Since it's a class used for little more than storage, with no
interesting non-magic instance methods, it makes sense for SinkConfig to
be a namedtuple.  The rest of its code was mostly just boilerplate,
after all.  What remains makes it do everything we need with only slight
changes to its behavior (most notably, SinkConfig must be instantiated
with all its arguments now).

They're now right next to boundary values, which is much more reasonable
than the arbitrary values they had before.

Added three new tests: test_set_tmpcfg_huge, test_set_tmpcfg_big, and
test_set_tmpcfg_neg.  They set temporary configurations with huge
(> UINT16_MAX), big (> 20 V, > 5 A), and negative voltages and currents,
respectively.  These tests all fail currently, so I'll work on making
them pass.

It's at 100% branch coverage.  Woohoo!

100% statement coverage for the SinkConfig class, except for __repr__
which is mostly for debugging anyway.